Day-02 database

Change table name

alter table Table name rename The new name of the table ;

alter table student rename student_info;

Change field name , type

alter table Table name change Name New column names data type ;

alter table student_info change student_name student_name int;

Add fields

alter table Table name add Column name type ;

alter table student_info add student_age int;

Delete field

alter table Table name drop Name ;

Change field type ( Try not to change )
alter table Table name modify Name New data types ;

Modify database character set

alter database Database name character set utf8

alter database 111 character set utf8

Be careful ： Need to restart mysql service , To take effect

Modify table character set

Alter table Table name character set utf8 collate utf8_general_ci

use student_info;
– Change table name
alter table student rename student_01;
– Change field name
alter table student_01 change name student_age int(11);
– Change the field name and data type
alter table student_01 change student_age student_name VARCHAR(255);

– Add fields

alter table student_01 add student_age int(11);

– Delete field
alter table student_01 drop student_name;

– Change field type
alter table student_01 modify student_age VARCHAR(255);

– Modify database character set
alter DATABASE student_info CHARACTER set utf8

– Modify table character set
Alter table student_01 character set utf8 collate utf8_general_ci
Constraint classification

Primary key primary key

Foreign keys foreign key

only unique

Non empty not null

Self increasing auto_increment

Default default

– Primary key constraint
–1 Create table time Add primary key
create table person(
id int,
name varchar(255),
income decimal(18,2),
primary key(id)
);
– Uniqueness
alter table person add unique(id);

–2 Create the table first Then add the primary key
create table person_01(
id int,
name varchar(255),
income decimal(18,2)
);
alter table person_01 add primary key(id);
conditional

and

And , and , It means , Commonly used in Two additional judgments must be met , Equate to java Medium &&

grammar :

select Column qualification from Table limit where A expression and B expression ;

Such as : Query the student table ,name It's Zhang San and the score is greater than 90 branch

select * from student where name=‘ Zhang San ’ and score > 90;

Only students who meet two conditions will be found

or

Or meaning , Commonly used in In the case of an added judgment , Equate to java Medium ||

grammar :

select Column qualification from Table limit where A expression or B expression ;

Such as : Query the student table ,name It's Zhang San or Results are greater than 90 branch

select * from student where name=‘ Zhang San ’ or score > 90;

As long as any one of the two conditions is met , Can

and Priority ratio or high

between … and …

stay … Between

grammar

select Column qualification from Table limit where Name between value 1 and value 2;

in

In the specified data

grammar

select Column qualification from Table limit where Name in( value 1, value 2…);

like

Fuzzy query

grammar :

select Column qualification from Table limit where Name like ‘ value ’ ;

order by Sort

grammar :

select Column qualification from Table limit order by Name asc/desc;

Asc : Ascending

Desc : Descending

limit Limit

grammar

select Column qualification from Table limit limit Number of pieces ;

select Column qualification from Table limit limit Starting value ( It doesn't contain ) , Number of pieces ;

Single table query

Group keywords group by

Common group functions

count(*) : Total number of articles

max( Field name ) : Maximum

min( Field name ) : minimum value

avg( Field name ) : Average

sum( Field name ) : The sum of the

create table student (
id int ,
name varchar(20),
teacher_id int,
score decimal(18,2) ,
primary key (id)
);
create table teacher(
id int ,
name varchar(20),
primary key (id)
);
insert into teacher (id,name)values(1,‘ Teacher zhang ’);
insert into teacher (id,name)values(2,‘ Teacher wang ’);
insert into student (id,name,teacher_id,score)values(1,‘ Zhang San ’,1,90);
insert into student (id,name,teacher_id,score)values(2,‘ Li Si ’,2,88.9);
insert into student (id,name,teacher_id,score)values(3,‘ Wang Wu ’,1,45.7);
insert into student (id,name,teacher_id,score)values(4,‘ Zhao Liu ’,1,84);
insert into student (id,name,teacher_id,score)values(5,‘ Xiao Ming ’,2,92.5);
insert into student (id,name,teacher_id,score)values(6,‘ Xiaohong ’,2,47);
grammar :
select count(),max( Field name ),min( Field name )… from Table name group by Field name ;
Such as : See how many students there are in the student list
select count() from student;
Such as : Check that the score in the student table is greater than 90 How many students are there
select count(*) from student where score > 90;
Group by

Such as : Query how many students each teacher brought ( Show teacher id that will do )

select teacher_id, count(*) as stu_count from student group by teacher_id;

Such as : Check the highest score of the students brought by each teacher

select teacher_id, count(*) as stu_count,max(score) as stu_max_score from student group by teacher_id;

Such as : Check the total score and average score of each teacher's students

select teacher_id, sum(score) as sum,avg(score) as avg from student group by teacher_id;
Union And union all
Merge query , Merge query results

Union Will remove duplicates

Union all Duplicates will not be removed

Such as : Query out teacher_id = 1 All the student information of

select * from student where teacher_id=1;

Such as : Query out The student's score is greater than 60 All the student information of

select * from student where score > 60;

Such as : Query out The student's score is greater than 60 or teacher_id = 1 All the student information of ( Remove duplication )

// use or Realization

select * from student where teacher_id=1 or score > 60;

// use union Realization

select * from student where teacher_id=1

 union

select * from student where score > 60;

Such as : Query out The student's score is greater than 60 or teacher_id = 1 All the student information of ( repeatable )

select * from student where teacher_id=1

 union all

select * from student where score > 60;

Common functions

– Show current version
select version();

– Display the current database
select database();

– Return the number of characters
select char_length(‘ After dinner ’);

– Returns the number of bytes occupied
select length(‘ War Within Three Kingdoms ’);

– String concatenation function return abc
select concat(‘a’,‘b’,‘c’);

– String concatenation function The first is the splice return a+b+c
select concat_ws(’+’,‘a’,‘b’,‘c’);

– Capitalize all letters
select upper(‘abcd’);

– Turn all letters to lowercase
select lower(‘ABCD’);

– Number of characters intercepted 1 Indicates which character to start with 3 Indicates how many are intercepted
select substring(‘ System information class ’,1,3);